The present invention relates to a control method for a fuel injection system of an internal combustion engine where an optimum injection timing or fuel injection pulse width can be set in temperature ranges of the engine in a completely warmup state from an extremely low temperature state.
In general, in a fuel injection control of this type, all cylinders are injected at the same time at the time of starting an engine to cope with a quantity of required fuel and to reliably start an internal combustion engine. A fuel injection pulse width to be set in this case is determined on the basis of an engine temperature (mostly detected from a coolant temperature) without parameters of factors for determining an engine state such as an engine rotating speed, an intake air quantity, etc., since the factors of the engine rotating speed, the intake air quantity are unstable.
On the other hand, in a fuel injection control after a stable combustion immediately after starting the engine, the fuel injection is transferred to sequential injections. Since the engine is cooled, hence a fuel injection finishing time is set to a relatively delayed crank angle of an intake valve opening range so as to prevent adherence of fuel to the wall of the intake port and valve, and a fuel injection starting time responsive to the fuel injection pulse width set on the basis of an engine rotating speed, an intake air quantity, an engine temperature, etc., is determined in accordance with the fuel injection finishing time as a reference.
Since the engine is cooled in the start of the engine from a cryogenic temperature state, fuel atomization is difficult, and hence the quantity of required fuel is increased. When a fuel injection pulse width responsive to the quantity of required fuel is set for all cylinder simultaneous injection, an injection is started in a certain cylinder after the intake is finished, or fuel is continuously injected even after the intake stroke is finished, and the quantities of fuel to be supplied to the cylinders are different. As a result, the quantity of fuel adhered to the wall is the high at starting timing. Hence a misfiring easily occurs in the cylinder in which the quantity of actually supplied fuel is small, and thus the engine starting not only becomes unstable, but also fuel consumption is increased due to adherence of fuel to the wall.
On the other hand, since it is difficult in the sequential injections to accurately decide the fuel injection timing because of unstable engine rotating speed at the time of starting, starting properties of the engine are impaired.
The engine temperature rises as the time lapses while warming up the engine after a stable combustion is performed. Therefore, atomization of fuel by the heat in an intake port occurs by injecting fuel when an intake valve is closed even in the warmup range. However, in the conventional engine in which the fuel injection finishing time is fixed, exhaust emission and fuel consumption are relatively deteriorated as the engine temperature rises.
Japanese Patent Application Laid-Open 60-11652 discloses a technique for improving fuel consumption, the output and the exhaust emission by variably setting a fuel pressure of fuel supplying to injector in response to a quantity of intake air and supplying fuel for obtaining an optimum combustion condition in an intake stroke range. A variation in the quantity of required fuel at the time of starting strongly depends on an engine temperature as compared with the quantity of the intake air, and hence it is difficult with this technique to obtain preferable starting properties and warmup performance after a combustion is performed in the engine.